Nuclear Magnetic Resonance (NMR) systems have been in use for many years and can be used to provide imaging and/or analysis of a sample being tested. For example, U.S. Pat. No. 6,160,398, U.S. Pat. No. 7,466,128, U.S. Pat. No. 7,986,143, U.S. patent application Ser. No. 12/914,138, and U.S. patent application Ser. No. 13/104,721 describe a variety of NMR technologies, and are incorporated herein by reference. Various different types of NMR include medical NMR, often referred to as Magnetic Resonance Imaging (MRI), and NMR for measuring properties of earth formations, which provides, for example, geophysical techniques for detecting properties of the earth's crust or earthen structures. This disclosure relates to the latter type of NMR, and so the term “NMR” as used herein refers to NMR in the geophysical context. While there is some overlap in the technologies that may be applied in MRI and NMR, the samples being measured and the environments in which measurements are performed are different, leading to many differences in the technologies applied.
In general, NMR measurement involves generating a static magnetic field within a sample volume, emitting Radio-Frequency (RF) electromagnetic pulses into the sample volume, and detecting RF NMR responses from the sample volume. Most commonly, NMR measurement involves emitting multiple RF pulses in rapid succession and measuring the RF NMR responses between the RF pulses. The measured RF NMR responses provide useful information about the sample volume.
NMR measurements may be used to estimate properties including, for example, the abundance of hydrogen contained within a sample volume as well as moisture content, porosity, permeability, and pore-size distribution of the sample volume. The measurement may also be used to determine fluid composition and fluid diffusion properties. NMR measurements may further be used to detect certain other atomic species, including carbon and potassium.
NMR has been applied as a geophysical technique using two primary approaches. In the first approach of downhole logging NMR, an NMR measurement apparatus is lowered into a borehole in the earth, and NMR measurements are performed to determine properties within and/or surrounding the borehole. The logging apparatus contains permanent magnets that create a static magnetic field for the NMR measurement, and one or more coils or antenna used to excite an NMR signal from fluids in the Earth formation and to measure this NMR response. A second approach, Earth's Field Surface NMR (EF-SNMR) utilizes Earth's natural geomagnetic field as the static magnetic field and one or more coils or antenna deployed on Earth's surface to excite and measure the NMR response of subsurface fluids.
While the two geophysical approaches have certain advantages, neither approach is ideal for rapidly obtaining information about the properties of the shallow or very shallow subsurface. The shallow subsurface is herein defined as any portion of the subsurface within about the upper 10 meters of the subsurface, that is, between the surface at zero meters depth to about 10 meters below the surface. The very shallow subsurface is herein defined as any portion of the subsurface within about the upper 2 meters of the subsurface, that is, between the surface at zero meters depth to about 2 meters below the surface. Logging NMR measurements offer high signal-to-noise, and because they produce a strong gradient in the static magnetic field, can use gradient imaging techniques to obtain information with high spatial resolution and precision. Logging NMR measurements, however, require the installation of a borehole and so cannot be used without disturbing the subsurface. EF-SNMR measurements are less invasive because they can be used to assess properties of a fluid bearing Earth formation without installing a borehole or well. EF-SNMR measurements, however, are limited by a very low signal-to-noise for two reasons. First, the NMR signal amplitude is proportional to the square of the static magnetic field strength and so is proportionally small in the Earth's weak magnetic field. Secondly, the NMR signal diminishes as the volume of material contributing to the measurement diminishes and so EF-SNMR measurements typically include measurements with large coils over large and deep volumes. Further, because the Earth's magnetic field is very uniform, EF-SNMR cannot take advantage of gradient imaging techniques and so may suffer from poor spatial resolution capabilities.